Self-ordered Porous Anodic Alumina Templates by a Combinatory Anodization Technique in Oxalic and Selenic Acids

A combinatory two-step anodization technique is presented to prepare self-ordered porous anodic alumina (PAO) templates. The first and second anodization steps are performed in oxalic and selenic acid at 40 V and 45 V, respectively, giving rise to 13-nm-diameter PAOs with high ordering and porosity of 1.4%. The anodization time for the second step is also changed to investigate the effect of the oxide layer thickness on the pore ordering, using field-emission scanning electron microscopy and fast Fourier transform analyses. Increasing the anodization time from 20 min to 1 h considerably improved the ordering and hexagonal domain structures. Further increasing the anodization time up to 8 h continuously deteriorated the ordering of the PAOs, although the nanopore growth rate remained almost constant. The fabrication of small-diameter PAOs in selenic acid is discussed and related to the electric field concentration at the oxide-electrolyte interface at the bottom of pores.